With this setup, I think we can use a builder:
```
ImmutableSet.Builder<String> ipsBuilder = ImmutableSet.builder();
if (allowedInterfaces.scanPublic) {
   ipsBuilder.addAll(node.getPublicAddresses());
}
if (allowedInterfaces.scanPrivate) {
   ipsBuilder.addAll(node.getPrivateAddresses());
}
ImmutableSet<String> ips = ipsBuilder.build();
checkState(!ips.isEmpty(), "node does not have IP addresses configured: " + node);
return FluentIterable.from(ips);
```